In a wireless communication system including multiple wireless devices and a wireless base station, the multiple wireless devices carry out communications via the wireless base station, and also carry out direct communications without an intervention of the wireless base station (e.g., refer to Patent Documents 1 and 2). For example, direct communications between wireless devices are referred to as device-to-device (D2D) communications. In contrast, communications via the wireless base station are referred to as cellular communications, for example. The 3GPP that is a standardization body that has standardizes the LTE, which is one of wireless communication systems, has been continuously conducting study of D2D communication technologies to add them to the LTE-A standard.
Patent Document 1: Japanese National Publication of International Patent Application No. 2012-507975
Patent Document 2: WO 2014/017498